The long term objective are: 1. To elucidate the molecular basis by which the new marine toxins, the latrunculins, affect the actin-based cytoskeleton. 2. To determine the relationships between the chemical configuration of the latrunculins and biological activities. 3. To use the latrunculins as a cytological probe of microfilament functions in normal and transformed cells, and as an in vitro probe of actin polymerization. Towards achievement of these objectives the research plan will involve chemical, biochemical and cell biological studies. At the chemical level the specific aims are: 1. To isolate and purify sufficient quantities of the natural toxins. 2. To develop synthetic derivatives of the latrunculins based on structure- activity relationships. 3. To develop labeled compounds for identification of the binding site. 4. To synthesize the entire molecule. At the biochemical level the specific aims are: 1. To determine the effects of latrunculins and their derivatives on polymerization of purified actin. 2. To examine the interaction between latrunculins and actin-binding factors or polymerization of purified actin. These aims will be achieved by investigating the kinetics and final extent of polymerization of purified actin in vitro utilizing spectrophotometric measurements. 3. To identify the latrunculin binding site using labeled compounds. 4. To determine the concentrations of actin in cells. At the cell biological level the specific aims are: 1. To evaluate the short- and -long term effects of the latrunculins and their derivatives on morphology, actin organization, division, and growth of normal and transformed cells. 2. To investigate the kinetics of the dynamic morphological and changes induced by latrunculins and cytochalasins. 4. To evaluate the effects of different culture regimens on the inactivation of latrunculin B by serum, and determine which part of the molecule is attached by serum. 5. To determine if latrunculins are concentrated in cells. It is hoped that these experiments will increase our understanding of actin polymerization in vitro and of the roles of the actin- based cytoskeleton in such key cellular processes as motility, shape determination, fertilization, cell division and growth, cell transformation, nerve cell differentiation and topographical organization of specialized surface membrane components.